AbstractThiosulfate–citrate–bile–sucrose agar (TCBS) has been shown to induce pathogenesis in the crown-of-thorns starfish (COTS), Acanthaster planci, providing potentially novel options to control COTS outbreaks, but the mechanistic basis for this effect is unknown. This study explores reactions of COTS to individual ingredients of TCBS, testing for allergic reactions versus pathogenesis. Four out of nine TCBS chemical ingredients tested induced allergic reactions and death in injected COTS. Peptone 10gl−1 and oxgall 8gl−1 induced 100% mortality, while yeast extract and agar induced death in 40% and 20% of COTS, respectively, 48h after injection. Peptone was evaluated at three different concentrations (10g, 5g, and 1gl−1). Peptone 10gl−1 induced 100% mortality, peptone 5gl−1 killed 60% of injected COTS, and peptone 1gl−1 induced death in only 20% of challenged COTS, indicating that toxicity of peptone is concentration-dependent. Sodium citrate induced moderate mucus production in all COTS challenged, but disease did not progress and COTS completely recovered after 52h. The remaining ingredients in TCBS did not produce any kind of clinical signs of disease. Peptone, oxgall, and yeast are potentially useful in controlling outbreaks because these protein extracts can be safer to use compared to previously used noxious chemicals. In addition, lowered concentrations are required to kill COTS, therefore increasing efficiency by saving time, money, and effort in COTS control programs.